Optics and Photonics Journal, 2017, 7, 101-108
http://www.scirp.org/journal/opj
ISSN Online: 2160-889X
ISSN Print: 2160-8881
Study of Plant Cultivation Using a
Light-Emitting Diode Illumination System to
Control the Spectral Irradiance Distribution
Atsushi Motogaito1,2,3, Naoki Hashimoto1, Kazumasa Hiramatsu1,2, Katsusuke Murakami4
Graduate School of Engineering, Mie University, Tsu, Japan
The Center of Ultimate Technology on Nano-Electronics, Mie University, Tsu, Japan
3
Iga Regional Satellite Campus, Mie University, Iga, Japan
4
Graduate School of Bioresources, Mie University, Tsu, Japan
1
2
How to cite this paper: Motogaito, A.,
Hashimoto, N., Hiramatsu, K. and Murakami, K. (2017) Study of Plant Cultivation
Using a Light-Emitting Diode Illumination
System to Control the Spectral Irradiance
Distribution. Optics and Photonics Journal,
7, 101-108.
https://doi.org/10.4236/opj.2017.76010
Received: April 20, 2017
Accepted: June 4, 2017
Published: June 7, 2017
Copyright © 2017 by authors and
Scientific Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
Abstract
In plant cultivation, the number of photons is more important than the light
energy from the chemical reactions that occur during photosynthesis. In addition, the blue and red photon flux (B/R) ratio is an important parameter for
plant cultivation. Here we discuss the effect of the spectral irradiance distribution and the B/R ratio on plant cultivation. We cultivated lettuce seedlings,
Lactuca sativa L. Cv. Okayama, using a light-emitting diode illumination system that can precisely control the spectral irradiance distribution and B/R ratio. The B/R ratio varied from 0.36 to 2.06 according to the intensity of the
blue light when the photosynthetic photon flux density values were sufficient
to ensure the 150 - 200 μmol∙m−2∙s−1. High photon flux densities of blue light
result in reduced plant length, plant height, and leaf area, thereby suggesting
its role in the suppression of leaf growth. Therefore, we conclude that a lower
photon flux of blue light (B/R Ratio) is optimal for lettuce cultivation.
Keywords
Plant Cultivation, LED Lighting, Blue and Red Photon Flux (B/R) Ratio
1. Introduction
Plant factories have many advantages, i.e., the plants’ growth environment (e.g.,
light, temperature, humidity, carbon dioxide concentration, moisture, and nutrients) can be controlled. Therefore, the high-quality planned production of
plants can be achieved. There are two types of plant factories. One uses sunlight
and another uses only artificial light. In this study, we focused on plant cultivaDOI: 10.4236/opj.2017.76010
June 7, 2017
A. Motogaito et al.
tion using only artificial light because it does not depend on the weather and can
be available in indoor spaces and/or in narrow spaces. There are several types of
light sources for plant cultivation such as high-pressure sodium lamps, metal halide lamps, fluorescent lamps, and light emitting diodes (LEDs) [1]. The use of
LEDs in plant cultivation has recently attracted attention due to a number of
advantages: (1) it is possible to precisely control the emission wavelength; (2)
their power consumption is smaller than that of other light sources, such as incandescent and fluorescent lamps; and (3) they are capable of pulse irradiation,
which is an advantage in plant photosynthesis. Mori et al. reported that the
growth rate of Lactuca sativa cv. “Natsuyo-Samdana” with pulse irradiation is
higher than that with continuous irradiation [2]. The chemical reaction in photosynthesis is as follows
H 2 O + CO 2 → 1 6 C6 H12 O6 + O 2
(1)
The reaction in Equation (1) depends on the number of photons. The action
spectrum of photosynthesis was first reported by McCree [3] [4]. Photons contribute to the generation of glucose, which determines the rate of photosynthesis.
Plants require visible light within a wavelength range of 400 - 700 nm to promote photosynthesis and growth [3] [4]. In particular, blue and red light wavelengths are necessary because they are absorbed by chlorophyll. In general, blue
light is helpful for morphological changes, whereas red light facilitates photosynthesis [5] [6]. In addition, Terashima et al. reported the role of green light in
photosynthesis [7]. Moreover, Fujiwara et al. reported plant cultivation using
LEDs of five peak wavelengths (405 nm, 460 nm, 630 nm, 660 nm, and 735 nm)
[8]. From these two reports, it can be seen that green light is necessary for plant
cultivation [7]. In plant cultivation, the number of photons is more important
than the light energy due to the chemical reactions that occur during photosynthesis; photons contribute to generating glucose, which determines the rate of
photosynthesis. Therefore, the photosynthetic photon flux density (PPFD),
which is the number of photons in the wavelength range of 400 - 700 nm, is often used. It is obtained from the spectral irradiation E(λ) (W∙m−2∙nm−1)of the
lighting system mentioned below [4]:
1 700
PPFD value μmol ⋅ m −2 ⋅ s −1 = ∫ E ( λ ) λ dλ
120 400
(
)
(2)
In addition, the blue and red photon flux (B/R) ratio is an important parameter for plant cultivation. Each plant species would have different optimum B/R
ratio, even though few studies have reported on the relationship between plant
cultivation and the B/R ratio. In our previous studies, this relationship was studied using a fixed spectral irradiance distribution with different B/R ratios, such
as high-intensity discharge lamps and LED lights [9]. We found that it was
possible to suppress leaf extension with a high photon flux density of blue light;
however, the plants’ leaves became hard and their thickness increased. Therefore, it is clear that controlling both the spectral irradiance distribution and the
B/R ratio is very important. Based on the above results, in this study, we culti102
A. Motogaito et al.
vated plants using an LED illumination system that could precisely control both
the spectral irradiance distribution and the B/R ratios to examine their effects on
plant cultivation.
2. Characterization of the LED Lighting System
The LED illumination system, measuring 500 × 350 mm2, was composed of surface-mounted LED arrays of blue, green, and red, as shown in Figure 1. The
power consumption was 140 W. The green and red LEDs were fabricated using
blue LEDs and phosphors. The LED peak intensity of three kinds of lights could
be adjusted from 0% to 100%, independently. The dominant wavelengths of the
red, green, and blue LEDs were 660 nm, 550 nm, and 460 nm, respectively.
The pulse frequency was kept constant at 2 kHz and the duty ratio of the onpulse could be varied from 0.3 to 0.7 (period: 500 μsec, on-pulse time 150 μsec 350 μsec). The intensity of pulse could be adjusted independently.
The spectral irradiance distribution was measured using an optical fiber and a
monochrometer. The distance (d) between the surface of the LED lighting system and the tip of the optical fiber was set between 150 mm and 250 mm to obtain PPFD values measured by JIS (Japanese Industrial Standards) lighting average method using light meter and quantum sensor (LI-250A and LI-190R, LICOR, inc.) of 150 - 200 μmol∙m−2∙s−1. The illumination conditions are listed in
Table 1. The intensity of the blue LED (B) varied from 10% to 100%, whereas
that of the green LED (G) varied from 40% to 100%, and that of the red LED (R)
was constant for all conditions to control B/R ratio by only the photon flux of
blue light. For all conditions, the on-pulse time was 350 μsec. For condition 4,
the photon flux of green light decreases to reduce the photon flux of the blue
light.
Figure 1. Top view of the LED illumination system.
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A. Motogaito et al.
Table 1. Illumination conditions.
#
Illumination Conditions
(For all conditions, R = 100%)
Photon flux for blue
light (μmol∙m−2∙s−1)
Photon flux for red
light (μmol∙m−2∙s−1)
B/R ratio
PPFD value
(μmol∙m−2∙s−1)
1
G = 100%, B = 100%, d = 150 mm
216.5
104.9
2.06
415.9
2
G = 100%, B = 100%, d = 250 mm
132.3
75.3
1.76
271.8
3
G = 100%, B = 20%, d = 150 mm
46.9
76.0
0.62
186.3
4
G = 40%, B = 10%, d = 150 mm
32.5
90.1
0.36
167.6
Figure 2 shows the spectral irradiance distribution at the center of the cultivation area. From the spectra, we can see that, when the intensity of the blue light
decreased, the spectral irradiance value also decreased. From these spectra and
the PPFD value, the value of the red and blue photon fluxes and the B/R ratio
can be calculated, as shown in Table 1.
Therefore, the B/R ratio changed from 0.36 to 2.06 depending on the intensity
of the blue light. Moreover, Figure 3 shows the distributions of the PPFD value
for condition #1 and #4. The PPFD values were almost sufficient to achieve the
150 - 200 μmol∙m−2∙s−1, which is required to cultivate the plants [10]. For each
case, the PPFD value, which is larger than 150 μmol∙m−2∙s−1 is maintained in the
cultivation area. However, there are some peaks of PPFD value because of the
differences in PPFD values at the center and the edge area due to the distribution
of the illuminance of each LED. Therefore, it is necessary to improve the distribution of the PPFD value.
3. Discussion of Plant Cultivation Using the LED Illumination
System
In this study, we cultivated lettuce seedlings of Lactuca sativa L. cv. Okayama.
The distance between the LED illumination device and the plants was 250 mm.
The lighting conditions are shown in Table 1. For all the conditions, the LED
lights illuminate the lettuce for 24 hours per day. The temperature of the cultivation area is almost 20˚C and the humidity is about 50%. We measured leaf
number (longer than 1 cm) and the area of whole leaves using scanned images,
stem lengths, stem diameters, plant heights, and plant lengths by destructive
measurements. Measurements were obtained on day 1, which was the first day of
light exposure, as well as on days 8, 12, and 15. The period of seedling rising is
12 days. Figure 4 shows photos of the leaves for conditions #1 and #4. The results demonstrate that high photon flux densities of blue light led to a reduction
in plant length and leaf area, therefore suppressing leaf growth.
Next, the changes in the plants’ fresh mass, dry mass, leaf number and area,
stem length and diameter, and plant height and length were characterized. Figure 5 shows the measurement results of the plant length, plant height, and leaf
area. On day 15, the leaf area, plant length, and plant height were reduced for
greater blue photon flux densities. When the blue photon flux density was excessive, the growth of the leaves was suppressed. The amount of blue photons in the
photon flux density has a large influence on the suppression of leaf growth. A
104
Spectral Irradiance (µWm-2nm-1)
A. Motogaito et al.
20000
Condition #1
Condition #2
Condition #3
Condition #4
15000
10000
5000
0
300
400
500
600
700
800
Wavelength (nm)
Figure 2. Spectral irradiance distribution at the center of the cultivation
area.
Figure 3. The distributions of the PPFD value for condition #1 and #4.
t-test, a type of statistical method, with a significance level of 5% demonstrated a
significant difference in the measurement result for the leaf area on day 15 for
conditions #1 and #4 using 5 samples in a staggered manner considering the
dispersion of the respective PPFD values. In addition to the leaf area, there were
also significant differences in the measurement results in terms of plant length,
and plant height.
Conversely, Figure 6 shows the measurement results for the stem length, stem
diameter, and leaf number. On day 15, the stem diameter was greater for increased blue photon flux densities; however, the stem length and the leaf number
did not have a difference as clear as the B/R ratio. When the amount of blue
photon flux density is insufficient, stem growth is suppressed. The amount of
blue photons in the photon flux density will therefore affect the stem growth.
From these results, it is thought that a high photon flux density of blue light
leads to a reduction in the plant length, plant height, and leaf area, therefore
suppressing leaf growth. Similar results were obtained by Oshima et al. in the
cultivation of red-leaf lettuce [10]. Therefore, we conclude that a lower photon
105
A. Motogaito et al.
Condition #1
5cm
5cm
Condition #4
5cm
5cm
Day 8
Day 15
Figure 4. Photos of the leaves for conditions #1 and #4.
Figure 5. Measurement results for the plant length, plant height, and leaf area.
106
A. Motogaito et al.
Figure 6. Measurement results for the stem length, stem diameter, and leaf number.
flux of blue light (B/R ratio: 0.3 - 0.6) is optimal for lettuce cultivation. Furthermore, in this study, to decrease the photon flux of blue light, the intensity of
green light was decreased. However, the effect of the green light must be considered in the future. This study was carried out in the perfect artificial cultivation;
however, these results can be extended to outdoor cultivations.
4. Conclusion
In this study, we discussed the effects on plant cultivation of the spectral irradiance distribution and the B/R ratios, which we precisely controlled using an
LED illumination system. The B/R ratio varied from 0.36 to 2.06 according to
the intensity of the blue light. The PPFD values were sufficient to ensure the 150
- 200 μmol∙m−2∙s−1 required to cultivate the plants. The lettuce was then cultivated using this LED illumination system. When the photon flux density of the
blue light was high, the plant length, plant height and leaf area were reduced.
Therefore, a high blue light photon flux density suppresses leaf growth. Because
we used blue LEDs in the illumination system, the B/R ratio which is less than
0.36 is not realized; therefore, we conclude that there is an optimum condition,
particularly lower B/R ratio, for lettuce cultivation.
Acknowledgements
The authors would like to thank Yoshikawa Co., Ltd. for providing the LED
lighting system. This work is partly supported by the TEPCO Memorial Foundation. The authors would like to thank Enago (www.enago.jp) for the English
language review.
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